The present invention relates to a communication apparatus, a communication system, and a communication control method.
In communications with employment of wireless LANs and power line communications using power lines, the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) system has been employed as one of packet collision avoiding methods.
In the CSMA/CA method, one communication apparatus which tries to commence a transmission checks whether or not signals transmitted by other communication apparatuses are present on a transmission line (concretely speaking, whether or not carriers are present which are utilized in communication on transmission line) in order that the first-mentioned communication apparatus can perform the transmission at this time. When the carriers are detected, the communication apparatus waits the transmission until the communications executed by other communication apparatuses are accomplished. After the communication apparatus has confirmed the accomplishments of the communications by other communication apparatuses, the first-mentioned communication apparatus further waits the transmission for a time calculated by adding an IFS (Inter Frame Space) time to a back off time determined at random, and then, commences the transmission after confirming that other communication apparatuses are not operated under communication by detecting carriers.
A back off time implies such a time required by that a communication apparatus which tries to commence a transmission monitors such a condition that other communication apparatuses are not operated under transmissions. FIG. 16 is a diagram for explaining the back off time. In FIG. 16, time periods 1610A, 1620A, 1630A, - - - , among signal frames 1610, 1620, 1630, - - - , which have been transmitted on a transmission line by any of communication apparatuses represent back off times. Back off times are sequentially determined by multiplying, for example, random values by a unit time (will be referred to as “unit back off time” hereinafter), while the random values can be determined by producing random numbers. There are many possibilities that this unit back off time is determined on the basis of such a time required for detecting a signal frame by a receiving apparatus (this time requires that, for instance, PHY layer detects signal frame, and MAC layer grasps detection of signal frame).
FIG. 17 is a diagram for representing one example as to a signal frame which is utilized when a conventional communication apparatus performs a communication. A signal frame 1700 has been constituted by a detection preamble field 1710, a synchronization preamble field 1720, a control field 1730, and an information field 1740. The detection preamble field 1710 contains 10 pieces of continuous detection preambles for detecting carriers. The synchronization preamble field 1720 contains signals (T1, T2) for synchronizations. The control field 1730 contains a signal (SIGNAL) for performing a communication control. The information field 1740 contains actual data (Data 1, Data 2, - - - ).
In FIG. 17, since a time length (t1 to t10) of one detection preamble is 0.8 μsec., a time length of all of the detection preambles is 8 μsec. Now, in such a case that a time required for that a receiving apparatus detects a signal frame is, for example, 8 μsec., 8 μsec. may be employed as a unit back off time. In this case, a length of a back off time becomes “8 μsec. X random value (integer).” It should be noted that a time required for that the receiving apparatus detects the signal frame is directly proportional to a symbol length used in a communication. This reason is given as follows: That is, in order that the receiving apparatus detects the signal frame within the shortest time period, a time defined by (1 symbol+processing delay) is required.
Conventionally, while a symbol length is a fixed length, the symbol length must be made long in order to firmly detect a carrier. On the other hand, if the symbol length is made longer, then the preamble length becomes long and also the back off time becomes long. As a result, there is no way to avoid that a transmission efficiency is deteriorated.
It should be understood that in order to improve transmission efficiencies of systems for performing communications by utilizing the CSMA/CA method, there are such communication systems which adjust upper limit values of back off times in response to available conditions of frequency bands under use with reference to, for example, JP-A-2005-12275.